1. Field of the Invention
This invention relates to a method and apparatus for the determination of the sulphur content in samples, especially samples having a weight ranging from 0.1 and 2 mg, which are submitted to analysis in order to quantitatively determine the presence therein of S or of different components, for instance C, H, N and S. In particular, the subject of this invention is a method and apparatus for determining the sulphur content, precisely measurable even when this presence is very reduced in quantity, in sample analysis carried out, in a known way, by means of combustion in a current of helium temporarily enriched with oxygen, catalytic oxidation and reduction of the combustion gases inside the same reactor, and gas chromatographic separation and detection of the outcoming products.
2. Description of the Prior Art
A method for simultaneous determination of C, H, N and S content in organic and inorganic samples according to the analytical procedure described above has been reported for instance in the publication Mikrochimica Acta 0/341-Dec. 2, 1977, E. Pella and B. Colombo. According to this method, the sample to be analysed is submitted to the above mentioned subsequent steps of combustion, oxidation and reduction, after which the resulting gases are conveyed to a gas-chromatographic column and then to a detector of the type exploiting the thermal conductivity principle (Thermal Conductivity Detector-TCD). As is known, the latter is capable of giving a diagram having several peaks separated from each other, each quantitatively indicating one of the components to be determined. In particular, the TCD detector is capable of giving in sequence a series of peaks indicating N.sub.2, CO.sub.2, H.sub.2 O and SO.sub.2 respectively. This system permits very precise and accurate detections of the different components indicated, and in particular it permits obtaining of a quantitative yield in SO.sub.2 despite the reactions which are carried out upstream the gas-chromatographic column. However, it has been noticed that, due to causes specific of the detection system, the peak indicating SO.sub.2 content gives an exact quantitative indication of sulphur content only when this content, or more precisely only when the SO.sub.2 content exceeds 0.5 .mu.g. In other words, the TCD detector response to SO.sub.2 is linear only for values higher than 0.5 .mu.g. Consequently, if the sample examined contains sulphur in very small quantities, these quantities are not detected by the usual TCD detector used in these cases, or are detected in an imprecise way which does not allow quantitative determination.
In any case, there are situations where it is necessary to detect even very small quantities of sulphur in the examined samples, both together with other mentioned components or alone.